BOSTON – A slim majority of voters support expanding the cap on charter schools in Massachusetts, a new poll shows.

The survey from the Western New England University Polling Institute suggests a proposed ballot question to allow up to 12 new charter schools or expansions each year would pass, if it is placed on the ballot in November.

Gauging the backing for the proposal from Great Schools Massachusetts, the poll of 497 registered voters conducted earlier this month found that 51 percent of respondents support the measure, while 26 percent oppose it and 23 percent are undecided.

Given that both sides of the charter school question are well-organized and have deep pockets, and that nearly a quarter of those surveyed said they are following the charter school issue “not very closely” or “not at all closely,” Tim Vercellotti, director of the WNE Polling Institute, said he expects opinions to shift as voters learn more.

“There seems to be a lot of room to maneuver in terms of supporters and opponents shaping public opinion on the charter school issue,” Vercellotti said in a statement. “There is the potential for these numbers to move in significant ways between now and November.”

The Senate last week passed a bill tying an increase in the cap on charter school enrollment with significant new funding for all types of public education. Charter school supporters dismissed the proposal and have said they intend to press on until the November ballot.

Great Schools Massachusetts, the coalition behind the campaign to authorize up to 12 additional charters annually outside the existing cap, has suggested it could spend up to $18 million in its campaign. Leaders of the Massachusetts Teachers Association have vowed to urge their union members to defeat the proposal should it land on the November ballot.